Catalytic converters containing various catalysts have been employed for years by automobile manufacturers to meet the ever-more-stringent regulations on emissions of hydrocarbon, carbon monoxide, and particularly, nitrogen oxides from internal combustion engines. Concurrently with the promulgation of these ever-tightening regulations on emissions, automotive engine manufacturers have worked to improve the fuel economy of such engines. This effort has led to the development of engines that operate with an excess of air beyond that which is required to consume stoichiometrically the fuel admitted to such engines ("lean-burn engines"). The advent of lean-burn engines has, in turn, exacerbated the problem of reducing emission of nitrogen oxides from internal combustion engines.
The selective catalytic reduction of nitrogen oxides by hydrocarbons has attracted much attention recently because it has the potential ability to remove nitrogen oxides from diesel exhaust and other oxygen-rich exhaust gases. A number of researchers have reported that cation-exchanged zeolites and other kinds of catalysts are quite effective in reducing NO.sub.x with hydrocarbons in an oxidizing atmosphere, but are quite susceptible to interference caused by water vapor. Another number of researchers has reported that catalysts comprising noble metal supported on alumina are effective for NO.sub.x reduction even in the presence of water vapor, but only in an extremely limited temperature range. Miyadera (Tatsuo Miyadera, Applied Catalysis B: Environmental, volume 2, 199-205, published in 1993) reported that a catalyst comprising silver on alumina has an unusual tolerance for water and is effective for NO.sub.x reduction over a broader temperature range, but did not investigate its performance in the presence of SO.sub.2. In this paper, on page 202, Miyadera states "[t]he silver on the Ag/Al.sub.2 O.sub.3 catalyst is supposed to be in the metal state under the reaction conditions, since silver oxide decomposes to metallic silver and oxygen at temperatures above 300.degree. C. Benton and Elgin [12] reported that water was not very strongly adsorbed on the metallic silver surface. Therefore it seems that water vapor has only a small effect on the reduction of nitric oxide over the Ag/Al.sub.2 O.sub.3 catalyst above 300.degree. C. Other catalysts such as Co/Al.sub.2 O.sub.3, Al.sub.2 O.sub.3, etc. consist of metal oxides and are subject to interference of water vapor because water adsorbs strongly on these oxides."
Contrary to the reports of these researchers and the teachings of the patents discussed below, the present inventor has discovered that catalyst comprising alumina and small silver oxide particles highly dispersed thereon is resistant to deactivation by SO.sub.2 and water vapor when used to reduce NO.sub.x, in exhaust gases from internal combustion engines.
A number of patents have been granted for compositions that superficially resemble that of the present invention. U.S. Pat. No. 3,857,921 (Tamura et al.) teaches a catalyst comprising iron and silver or their oxides plus a third metal for elimination of NO.sub.x in exhaust gases.
U.S. Pat. No. 5,300,269 (Aichinger et al.) teaches a catalyst comprising metallic silver on alumina for decomposing dinitrogen monoxide.
U.S. Pat. No. 5,208,202 (Muramatsu et al.) teaches a catalyst comprising an alkali metal, a Group IB element and a rare earth element.
U.S. Pat. No. 5,407,651 (Kawai) teaches a catalyst comprising a zeolite having cobalt and silver contained therein to be effective as a three-way catalyst for an oxygen-rich exhaust gas.
More closely on point, U.S. Pat. Nos. 5,534,237 and 5,589,432 (Yoshida et al.) teach that a catalyst comprising alumina impregnated with silver or its oxide is effective for NO.sub.x removal from an oxygen-rich exhaust gas and that pre-treating the catalyst with SO.sub.2 further increases the removal ratio of NO.sub.x. However, both Yoshida patents are indifferent as to whether the silver exists as a metal or its oxide and teach nothing about the effectiveness of the catalyst when the exhaust gas contains SO.sub.2.
Finally, U.S. Pat. No. 5,559,072 (Itoh et al.) teaches that a catalyst comprising activated alumina and silver dispersed thereon is effective for NO.sub.x removal from an oxygen-rich exhaust gas. As in the Yoshida patents, however, the silver may be in either the metal or the oxide form and nothing is taught about the effectiveness of the catalyst when the exhaust gas contains SO.sub.2.